216 ELEMENTS OF ELECTRICAL ENGINEERING

motor as function of the impressed voltage at constant output,
that is, constant product, current times generated e.m.f. If
i = current and P = constant output, the generated e.m.f.

P
must be approximately ei = -•> and thus the terminal voltage

& = €l _|_ fa Proportional hereto is the field excitation FQ.
The resultant m.m.f. of the field is thus F = F0 — iq, and corre-
sponding thereto from curve A in Fig. Ill is derived the e.m.f. e0
which would be generated at constant speed by the m.m.f. F.
Since, however, the generated e.m.f. must be e^ the speed is

BI
changed in the proportion --•

m

REV.

X
^

M»N.
£000 1800

X
X

x

*
X

X
s

1200 1000
800

/

D

/

Ul
a

/
/

GOO 400 200

/
r

V
_X

voi-\
s*

20 40 60 80 100 ' 120 140 160 180 200

FIG. 120.—Shunt motor speed curve, variable impressed e.rn.f.

The speed rises with increasing and falls with decreasing im-
pressed e.rn.f. Still further decreasing the impressed e.m.f.,
the speed reaches a minimum and then increases again, but the
conditions become unstable.

Series Motor

76. The speed characteristic of the series motor is shown in
Mg. 121 at constant impressed e.m.f. e. A is the saturation
curve of the series machine, with the current as abscissas and at
constant speed. At current i, the generated e.m.f. must be

e - ir, and the speed is thus ^~ times that, for which curve A

&\

is plotted, where tl = e.m.f. taken from saturation curve A.